IMPROVING COGNITIVE FUNCTION IN RODENTS VIA INCREASING GLYCINE LEVELS IN BRAIN BY THE NOVEL GLYCINE TRANSPORTER-1 INHIBITOR BI 425809

Abstract

Dysfunction of the glutamatergic system related to the NMDA receptor may play an important role in cognitive impairment in Alzheimer’s disease. One approach to counteract this is the extracellular increase of the NMDA receptor co-agonist glycine by glycine transporter-1 (GlyT1) inhibition. Thus, inhibiting GlyT1 has potential for treating memory impairment in Alzheimer’s disease via strengthening glutamatergic neurotransmission. Previous studies with GlyT1-inhibitors show memory-enhancing effects in animals. This study characterizes the potency and selectivity of the novel GlyT1 inhibitor BI 425809 and its effects on glycine increase in rat cerebrospinal fluid (CSF). BI 425809 was also evaluated in two rodent cognition tasks assessing episodic memory and working memory. The molecular potency of BI 425809 for GlyT1 was determined by inhibition of [3H]-glycine uptake in human SK-N-MC cells and rat primary cortical neurons. Selectivity was tested against GlyT2 and other off-targets by uptake and receptor binding assays, respectively. Concentrations of glycine in rat CSF samples collected from cisterna magna following oral administration of BI 425809 were determined by HPLC-MS/MS technique. BI 425809 was tested after oral application in the mouse T-maze spontaneous alternation test and in the social recognition test in naive rats. The IC50 value of BI 425809 on GlyT1 was 4.0 nM in SK-N-MC cells and 4.4 nM in rat primary neurons. BI 425809 demonstrated no relevant activity against GlyT2 (IC50 >10 μM) and 103 off-targets at 10 μM. A dose-dependent increase of glycine in rat CSF was observed. BI 425809 reversed MK-801-induced memory deficits in the mouse T-maze (working memory) and improved memory performance in the rat social recognition task (episodic memory). BI 425809 is a potent and selective GlyT1 inhibitor. Systemic administration of BI 425809 led to an increase in glycine levels in rat CSF demonstrating functional target engagement – i.e. GlyT1 inhibition in the brain – showing that glycine levels in CSF can be used to assess GlyT1 inhibition centrally. This might also be used to evaluate central target engagement in clinical trials. BI 425809 showed memory enhancing effects in animal cognition tests. GlyT1 inhibition may be a potential approach to pharmacologically improve cognition in Alzheimer’s disease.